e-PS, 2008, 5, 17-28
ISSN: 1581-9280 web edition
ISSN: 1854-3928 print edition
e-PRESERVATIONScience
www.Morana-rtd.com
© by M O R A N A RTD d.o.o.
published by
M O R A N A RTD d.o.o.
A PORTABLE MICRO-FADING SPECTROMETER FOR
FULL PAPER
VERSATILE LIGHTFASTNESS TESTING
Andrew Lerwill *1,2 , Joyce H. Townsend 1 , Haida Liang 2 ,
Jacob Thomas 1 , Stephen Hackney 1
1. Conservation Department, Tate,
Millbank, London, SW1P 4RG, U.K.
2. School of Science and Technology,
Nottingham Trent University,
Nottingham, NG11 8NS, U.K.
corresponding author:
andrew.lerwill@tate.org.uk
received: 22.01.2008
accepted: 15.04.2008
key words:
Micro-fading, Lightfastness, Fadometer,
Watercolour materials, Spectroscopy,
Accelerated ageing
The design and experimental method for the use of a novel
instrument for lightfastness measurements on an artwork
is presented. The new micro-fading spectrometer design
offers increased structural stability (which enables portability) and increased versatility over the previous, published
design, broadening the scope of locations at which data
can be acquired. This reduces the need for art handling or
transportation in order to gain evidence-based risk assessments for the display of light-sensitive artworks. The
instrument focuses a stabilized high powered xenon lamp
to a spot 0.25 mm diameter (FWHM) while simultaneously
monitoring colour and spectral change. This makes it possible to identify pigments and determine the lightfastness
of materials effectively and non-destructively. With
2.59 mW or 0.82 lumen (1.7· 10 7 lux for a 0.25 mm focused
spot) the instrument is capable of fading Blue Wool 1 to a
measured 11 ΔE ab value (using CIE standard illuminant D65)
in 15 min. The temperature increase created by focused
radiation was measured to be 3 to 4 °C above room temperature. The system was stable within 0.12 ΔEab over 1 h and
0.31 ΔE ab over 7 h . A safety evaluation of the technique is
discussed which concludes that some caution should be
employed when fading smooth, uniform areas of artworks.
The instrument can also incorporate a linear variable filter.
This enables the researcher to identify the active wavebands that cause certain degradation reactions and determine the degree of wavelength dependence of fading. Some
preliminary results of fading experiments on Prussian blue
samples from the studio materials of J. M. W Turner (17551851) are presented.
1
Introduction
Those responsible for the world’s cultural treasures within museums
have a duty to preserve these works whilst allowing public access.
Often these two requirements result in museum policy being driven in
opposing directions. The necessity of illumination for the display of
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photo-sensitive works of art is an example of this
impasse. Therefore, the application of technology
to solve issues in the conservation and display of
works of art warrants further investigation.
2
Instrument Design and
Performance
2.1
Instrument Design
To determine the safety of display and effectiveness of display policy, a novel micro-fading spectrometer has been designed and constructed taking inspiration from the Whitmore design 1,2 and its
application 3,4,5 . An instrument was constructed
that was capable of identifying materials more light
sensitive than Blue Wool #2 through direct fading
in artworks on a sub-millimetre diameter spot such
that the faded spot is not discernable by the viewer. Fading and colour change are carried out
simultaneously.
The instrument developed is approximately half
the cost of the previous published design. It is a
flexible, compact, lightweight and mobile instrument which removes the need for transportation of
art work and unnecessary art handling (Figures 1
and 2). It can function in two modes of operation:
firstly, as a transportable compact microfading
spectrometer capable of identifying the sensitivity
of artifacts to visible light exposure, and secondly,
with a linear variable filter to increase the scope of
investigation beyond that of the broad spectrum.
The latter application is discussed further in section 3.
It is intended that some improvements on this
design will increase the portability and ease with
which a researcher can conduct micro-fading, so
the need for object transportation is reduced, and
therefore the rate and also scope of locations at
which data can be acquired increased.
To increase the accuracy of colour measurements
and improve the ease of application, some
changes to the previous design were implemented.
An improvement of the precision of probe positioning relative to the sample, a good homogeneity of
illumination across the faded area, a controlled
intensity at the illuminated surface from the lamp,
and an improved ease of confocal probe alignment
were amongst the changes. A reduction in the
heating of the sample area by the illuminating spot
was also achieved and a method of documenting
the exact location of fading on an artwork is being
developed.
The instrument also differs from a previous design
by Paul Whitmore et al in that a linear variable filter system was added, which enables assessment
of the wavelength dependence of fading and
broadens the scope of information that can be
acquired regarding fading of artist’s materials.
Tests are carried out on sub-millimetre diameter
spot size while at the same time monitoring
change in the reflectance factor of the sampled
region. To do this the monitored spectrum is converted using the Commission International de
l’Eclairage (CIE) 1976 L*a*b* equation for the 2 o
standard observer under the standard illuminant
D65. Via this method an automated calculation of
colour difference of the fading spot is monitored in
real time in ΔE ab units.
18
For use as a microfadometer, a high-powered continuous-wave xenon light source (Ocean Optics
HPX2000) is connected directly to a solarization
resistant optical fibre with a 600 micron fibre core.
The end of this fibre is connected to a confocal
probe designed for this task, containing two
matched achromatic pairs. Light passes through
an extended hot mirror utilized to remove the
infrared in order to reduce temperature and the
ultraviolet to better simulate the museum environment (Figure 3). The filtered light is focused to a
0.25 mm spot by the matched achromatic doublet
pair on the sample surface.
It is possible to move the location of the lenses in
the probe as they are contained within lens tubes
on adjustable mounts. Adjusting the position of the
first lens (from the light source) alters the working
distance and the size of the focused spot size. To
a certain extent it is possible to do this without significantly altering the fading rate. This is because
moving the first lens closer to the fibre output couples more light to the spot which compensates for
the increase in fading area. This possible alteration of the instrument can lead to increased sampling area, and therefore data that is more representative of varied and highly-textured surfaces.
This would be useful for example when fading
reconstructed paint samples rather than actual art
work where a small faded area is not an important
safety measure to prevent visible damage.
In order to monitor colour change, scattered light
from the small sample area is then coupled back
into the optical system via another optical probe of
the same design at 45 degrees to the normal.
Sampled radiation then passes through a neutral
density filter to avoid saturation of the fibre optic
spectrometer. The spectrometer (Avantes Avaspec
2048) receives this signal via an optical fibre, and
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
© by M O R A N A RTD d.o.o.
Figure 1: A schematic representation of the Microfadometer, including linear variable filter.
the software (AvaSoft 7.0) analyzes change in the
spectrum and the rate of fading occurring in real
time.
Figure 2: The instrument fading a sample.
The probe is mounted on an XYZ stage capable of
sub-micron scale movements. The Z axis stage is
motorized. It is therefore possible to achieve fine
alignment of the probe with the surface remotely
rather than leaning over the artwork. This in turn
enables adjustment when the probe is beyond
arms-reach, e.g. over an art work when the probe
is mounted on a gantry to enable movement over
the surface of an artwork that is laid flat. This is
also an important aspect of the design, as it
becomes possible to achieve best focus remotely.
To achieve best focus, the software gives the integrated counts from the reflected spectrum over the
full spectral range (400 to 700nm), which enables
fine adjustments undetectable when aligning by
eye. By making small incremental adjustments in
position that would not be possible using a manual micrometer screw, it is possible to define best
focus to a greater accuracy.
Figure 3: The measured transmission of the extended hot mirror
used to filter the incident radiation of the instrument.
Future efforts to develop the instrument will
include attaching a webcam to the probe which will
enable a record of the location of fading on an artwork to be recorded. As well as this, an automated
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
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fadometer system is in development which will be
used to produce large amounts of data for a variety of samples and enable greater throughput to
more accurately categorize the behaviour of a
larger number of samples.
2.2
Probe Alignment
To ensure confocality, both probes were illuminated with low intensity radiation and focused onto a
CCD chip (Figure 4). For easier analysis, in Figure
4c the red area indicates the sampling area of the
receiving probe and green the illuminating area
(this creates a yellow overlap). The yellow region
indicates where both fading and colour monitoring
takes place.
When correctly aligned, it was shown that best
focus of the probe provides the maximum signal to
the spectrometer, and ensures reproducible spot
size. Failure to align correctly leads to the probe
focusing incorrectly, which can lead to a large variation in the calculated fading rate.
To fade a sample, the instrument operates as a
reflectance spectrometer with a high powered light
source. In order to make reflection measurements,
a dark spectrum and reference spectrum are
acquired. The reference spectrum is recorded on a
polished barium sulphate sample.
A neutral density filter is used to reduce the beam
to a level where best focus can be obtained without a significant level of radiation being incident on
a
b
c
Figure 4: Images produced in focusing the instrument probes
onto a CCD chip (a) illuminating probe, (b) sampling probe, (c)
illuminating and sampling probe (a and b) combined in alignment.
Figure 5: The relative power spectrum of incident radiation used
in broad spectrum fading tests.
20
Figure 6: The total counts of the spectrometer from 400 nm to
700 nm over 400 min.
the sample. The probe is adjusted on the sample
in order to come to best focus and acquire maximum reflected intensity on the object in the
desired location. The shutter of the lamp is then
used to stop illumination as the neutral density filter is being taken out in preparation for fading.
Colour differences are monitored in real time using
the spectrometer software in order to prevent fading beyond acceptable levels which have been
independently determined in the development
process.
2.3
Light Source Behaviour
The instrument produces 2.59 mW or 0.82 lumen
(1.7·10 7 lux for a 0.25 mm focused spot). The relative power spectrum of light incident on the sample measured using a calibrated spectrometer is
shown in Figure 5. The xenon bulb output will alter
as it ages and this makes it necessary to monitor
probe output regularly.
If it is desired to compare the time a sample undergoes fading using the fadometer to years in a
gallery setting, we need to assume the sample
would fade to the same degree independent of the
rate in which it is faded (or that reciprocity holds 6 ).
At 50 lux for 8 h per day 7 days per week, the fading rate of the instrument can be considered as 1
min approximately being equivalent to 2 years in a
gallery setting assuming reciprocity holds over 5
orders of magnitude. We will examine the issue of
reciprocity for a range of fugitive pigments in a
separate study. Unlike conventional fading, the
microfadometer is capable of testing reciprocity
over at least 4 orders of magnitude. Other limitations of the technique which prevent more certain
statements in this vein being made, such as difference in the spectral power distribution between
gallery lighting and the xenon lamp of the instrument and sample colour reversion are discussed
by Whitmore et al 2 .
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
© by M O R A N A RTD d.o.o.
Analysis of the stability of the system took place
over 400 min using illumination of barium sulphate
as a non fugitive reference over this period.
Variation at any wavelength from 410 to 720 nm
was within 1.5% with the majority within 1% variation.
Total counts of the spectrometer at all wavelengths increased 1.1% over the period (Figure 6).
The dark current over 7 h was constantly monitored and subtracted by the spectrometer software.
The stability of the system was shown to produce
an error no greater than ΔE ab of 0.31 at any reading over the 7 h period. The initial hour produced
no more than a ΔE ab of 0.12 at any reading.
2.4
Probe Position Sensitivity
In order to determine empirically the diameter of
the area that would be faded by the incident light,
the focused spot of the probe was analyzed by
observing its alteration through focus using a
CCD. FWHM values were taken when varying the
working distance of the probe to the CCD (Figure
7) which gave a spot size on the CCD chip of 33
pixels or 0.25 mm. It was verified that 1 pixel width
Figure 7: The FWHM of the focused spot profile through focus in
5 µm increments.
was 7.5 µm as per manufacturer specifications.
From this technique it was possible to determine
the spot size diameter to 1 pixel or 6% of the fading area. Figure 7 illustrates that the diameter of
the spot did not alter for 50 µm through focus.
The effect that small errors in focusing have on
received signal and colour measurements was
investigated. The sensitivity in positioning of the
probe relative to the surface being sampled was
determined by calculating relative ΔE ab at various
locations through focus, compared to values
L=100 a=0 b=0. This provided an illustration of
how a small change in probe position, (for example
relaxation of the probe holder, or altering of the
sample/probe geometry in repositioning the probe
from the white target to sample) can create error in
measured colour.
The relative colour difference was measured moving the probe in 50 nm increments through focus
along the optical axis when illuminating a polished
barium sulphate white tile. Colour data readings
are shown in Figure 8, demonstrating that the
colour measurements did not alter for 40 µm
through focus. From this analysis, colour measurement is shown to be more sensitive than the variation in size of the illuminated spot with probe position, as it is required not only that the spot be
focused but also the two probes be aligned.
In order to investigate the accuracy of colour
measurement using the instrument, reflectance
standards were used to investigate the accuracy of
the instrument in comparison to results from other
colour measurement techniques. This indicated
that some standards, although accurate and reliable for use with colour measurement instrumentation that use relatively large sampling areas, these
standards are less accurate on the scale of measurement of the instrument under discussion (0.25
mm). It was found a significant variation in
reflectance over the surface can be observed over
the sub-millimetre scale for all reflectance standards.
2.5
Figure 8: Alteration of colour measurements from microfadometer probe movements in 50 nm increments through focus represented in relative ΔE ab values when measuring a white tile and
comparing the colour measured with L=100 a=0 b=0.
Sample Visibilty and Size
A series of faded spots were produced ranging
from ΔE ab 1 to ΔE ab 8 on both Lightcheck ULTRA
and Lightcheck Sensitive (Figure 9). Lightcheck is
made of a light sensitive coating printed onto a
paper substrate. The colour changes of Lightcheck
indicate the degree of exposure. These samples
were chosen as they provide an approximation to
a worst case scenario in that they both provided
very smooth highly fugitive surfaces. With both
types of sample, it was possible to observe many
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
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a
b
Figure 9: Colour change produced using the Microfadometer on
Lightcheck ULTRA (A) and Lightcheck Sensitive (B).
spots in the series. It was found that 5 different
observers if shown the location could see spots
down to a colour difference of 2 or 3 ΔE ab units on
a pristine Lightcheck surface under good lighting.
Importantly it was found that in situations where
the Lightcheck surface was altered to reduce uniformity, for example by folding to vary the surface
texture, it was impossible to see to such low levels
of damage.
Practically speaking, when fading rougher, more
textured, varied surfaces, for example when fading
samples of oil paint on canvas it is possible at
times to fade to ΔE ab of 15 and more and not
observe any alteration as has been previously
considered the case. 1 This indicates that the damage is hidden by the texture in which it exists and
can be visible even at such low levels of fading.
These findings are further verified when fading
watercolours. It is possible at times to fade to ΔE ab
15 and beyond and observe no change visually.
However this depends on the uniformity of the surface. Importantly, on many samples which were
very uniform, such as various Prussian blue samples a fade of 5 to 6 ΔE ab was visible on close
inspection and often also at reading distance (25
cm).
22
After fading the series it was possible to image the
damage profile of each spot. An image of a spot
faded to a colour difference of 5 ΔE ab on
Lightcheck Sensitive was captured using a calibrated microscope. Analysis showed good uniformity of illumination and fading across the focal
region. As well as this, a typical example of the
measured normalized profile of a 5 ΔE ab faded
spot, and a measured normalized profile of the
incident illumination at best focus was also compared and shown to match well (Figure 10). The
microscope camera was calibrated to 800 pixels
per mm and this showed a variation in the FWHM
spot size dependent on the degree to which we
faded. This ranged from 0.22 for ΔE ab of 2 to 0.25
for ΔE ab of 8. A separate investigation of spot size
up to 16 ΔE ab showed that continued fading led to
continued increase of FWHM spot size.
3
Wavelength Tunable System
3.1
Technique Introduction
Colourants that are regarded as fugitive are faded
predominantly by the visible region, 7 therefore the
effect of visible radiation of different wavelengths
on deterioration of fugitive pigments warrants further investigation via a wavelength tunable system.
It is possible to filter the xenon lamp of the instrument using linear variable filters (Ocean Optics
LVF-UV-HL and LVF-HL) to move through the
desired wavelength range, and shape the fading
spectrum. The filter bandwidth of this technique is
20 to 30 nm FWHM and it is possible to vary the
central wavelength of the filter in the visible range
(Figure 11)
In previous efforts to investigate the wavelength
dependence of fading, Aydinli, Krochmann et al . 8
and McKlaren 7 divided the visible spectrum into 3
wavelength sections to observe the relative
degree of damage. In later work by Kenjo, 9,10 the
number of divisions increased to seven wavebands (located from 390 nm to 700 nm) on six different colorants. Similarly, Saunders and Kirby 11
used broad band interference filters with bandwidths of 70 nm, peak transmittances at 50 nm
intervals in the visible range from 400 nm to 700
nm.
Building on this work and utilizing new apparatus,
the wavelength dependence of fading of many pigments and samples can be investigated further
and at a greater resolution than previously
attempted. This will be done to highlight active
wavebands and determining the wavelength speci-
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
© by M O R A N A RTD d.o.o.
a
Abitrary units
b
distance /mm
Figure 10. (a) An image of a spot faded to a colour difference of 5 ΔE ab on Lightcheck ULTRA captured using a calibrated microscope.
(b). A typical example of the measured normalized profile of a 5 ΔE ab faded spot (indicated by the continuous line) and a measured normalized profile of the incident illumination at best focus (dashed line).
ficity of degradation caused by the specific visible
regions for light sensitive materials.
3.2
Experimental Method
The tunable instrument operates in a similar way
to that described previously. The spectra must be
recorded before and after fading in order to obtain
a colour difference value.
After an initial reading has been taken, the variable filter is adjusted to the chosen wavelength
prior to fading the sample. The filter is then
removed after fading to take a spectral measurement. Due to the presence of the filter, colour
measurements are not possible during fading
unless the shutter is opened, the filter removed,
and a measurement rapidly taken in order not to
alter the degree of fading.
In order to gain information on the wavelength
dependence of fading, the variation in power with
wavelength of the instrument must be compensated for. This is because neither the transmission of
the filter system (Figure 11), nor the power of the
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
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Figure 11: Tunable filter transmission at various wavelengths typically employed by the tunable instrument.
xenon source at various wavelengths is constant
(Figure 5).
The addition of the variable filter holder increased
the distance between the light source and the
fiber, thus reducing the incident power to 1.46 mW
or 0.46 lumen at focus (without the variable filter).
This reduction in incident power leads to a reduction in fade rate.
It was found that fading spot size remains at
0.25mm when sampled using a CCD at intervals
from 400 nm to 700 nm using the same technique
as discussed in section 2.4.
in just over 5 min and Blue Wool 2 to the same
level in twice that time period as expected.
4.2
Temperature Increase
In order to evaluate the safety of the instrument for
use on artworks and to know to what degree temperature may play a part in any observed results,
it was necessary to quantify the temperature
increase caused by the focused radiation. Two
techniques were employed. On separate occasions two different thermocouples were coated
The technique of initially monitoring the sample
lightfastness using a broad spectral fade enables
the user to determine a suitable length of time to
fade the sample. A half hour period has typically
been used to fade samples as fugitive as Blue
Wool 1 to 2. Results from this technique are presented in section 4.3.4.
4
Results and Discussion
4.1
Rate of Fading
ISO Blue Wool Standards are an internationally
accepted method of measuring fading within the
conservation community. Eight different degrees
of lightfast dyes can be used (with 1 being the
least lightfast to 8 the most). The effect of fading
Blue Wool samples 1, 2, and 3 by focusing 2.59
mW to a 0.25 mm diameter area can be seen in
Figure 12. This illustrates that the instrument is
capable of fading Blue Wool 1 to a ΔE ab value of 7
24
Figure 12: The fading rates of Blue Wool 1, 2 and 3 for the instrument when fading using the broad spectrum.
Figure 13: A photograph of a liquid crystal thermometer immediately after being irradiated with the microfadometer. Note 22 is
clearly visible at 22 °C room temperature and the bottom right
hand corner of the 26 showing a small circular area which has
been increased in temperature to approximately 26 °C by the
focused probe.
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
© by M O R A N A RTD d.o.o.
Samples of Prussian blue pigment (Tate Gallery
Archive 7315.7#6) from the studio materials of J.
M. W Turner (1775-1851, the materials dating from
the end of his life) underwent analysis using the
instrument in both modes of operation previously
discussed.
4.3.1
Figure 14: Some initial results of fading experiments on Prussian
blue pigment samples from the studio materials of J. M. W
Turner. A note for clarity is these results were created using the
instrument without the use of a variable filter as defined in section 3.
with various light and dark paint samples on paper
and illuminated by the focused spot. A thermocouple was also lightly coated with a variety of paints
as well as exposing the bare sensing junction.
The same temperature increase of 3 °C to 4 °C
above room temperature was observed in all
cases.
As a second method, a thermometer that contains
heat-sensitive (thermochromic) liquid crystals that
change colour to indicate different temperatures
was used. A number in a series corresponding to
the environmental temperature becomes translucent when it is reached. By focusing the probe
onto the various temperature-sensitive numbers,
26 °C clearly altered whereas all others from 12 to
34 (increments of 2 °C) did not. The area heated
by the radiation remained briefly unaltered after
the light was removed by a shutter, before cooling.
A photograph of this can be seen in Figure 13.
4.3
Application to Prussian Blue
Pigment
One pigment has consistently been reported to be
phototropic (that is, to lose colour due to light
exposure, and to regain it in the dark): Prussian
blue, ferric ferrocyanide, iron(III)hexacyanoferrate(II),
conventionally
represented
as
Fe 4 [Fe(CN) 6 ] 3 ·xH 2 O. The formula quoted by
Berrie 12 is more correct: M I Fe III Fe II (CN) 6 ·nH 2 O,
where M I is a potassium (K + ), ammonium (NH 4+ ) or
sodium (Na + ) ion, and n=14-16. Reports of fading
in light in the presence of normal air and/or nitrogen have been summarised by Kirby 13,14 and
Rowe. 15 Complete colour loss under hydrogen was
noted by Russell and Abney. 16,17 Reduction of
Fe(III) to Fe(II), a reversible reaction, is the cause.
The Effect of Water Dilution
The effect of water dilution of the Prussian blue
sample in gum Arabic medium was investigated.
Painted samples on filter paper were prepared
from an undiluted stock suspension of Prussian
blue in gum Arabic (Neat), 1 part Prussian blue
sample with gum Arabic to 1 part water (1 to 1)
and 1 part Prussian blue sample with gum Arabic
to 5 parts water (5 to 1) in various dilutions
through to 1 part Prussian blue sample with gum
Arabic to 100 parts water (1 to 100). The results of
the rate of fading can be seen in figure 14. Fading
rate is dependent on the intensity of the colour
wash: a very dilute wash does not cover all of the
paper substrate with pigment particles, and as the
pigment is the most light sensitive component,
increasingly pigment-rich samples fade more rapidly.
4.3.2
The Reversion of Colour
Colour reversion of Prussian blue samples in the
dark was also investigated over a period of 2 days
with 2 colour reversion periods. The sample was
fixed in position and then faded. During the 2
reversion periods (14 and later 10 h) the sample
was kept in darkness by housing it to remove incident light. The probe was not moved during the
experiment. Colour measurements during the fading process along with changes due to reversion
are represented in Figure 15.
The pigment became less sensitive to the exposure of light after the initial cycle of fading. Further
investigation is necessary to establish how long
(or if at all) it takes for the reversion in the dark to
bring back to its original value. As previously stated a reversible reaction between Fe(III) to Fe(II) is
the cause of this reversion.
4.3.4
The Wavelength Dependence of
Fading
The degree of fading by filtered radiation from 400
nm to 700 nm (bandwidth 20-30 nm FWHM) was
investigated in increments of 25 nm through the
visible region for a neat Prussian blue sample. The
length of time with which we faded was altered at
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
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Figure 15: The reversion of Prussian blue pigment over a 2 day period, with 2 pauses of 14 and later 10 h during which colour reversion
of the pigment took place in the dark.
each wavelength. This was done in order to compensate for the variation in incident power with
wavelength caused by the spectral power distribution of the lamp (Figure 5) and varying transmission of the filter at each wavelength (Figure 11).
This resulted in a power distribution at focus as
illustrated below in Figure 16.
change as a function of wavelength for the same
amount of incident energy at each wavelength.
The action spectrum shows that the blue end of
the spectrum causes more damage than the red
part of the spectrum. Further results of this type
applied to other samples will be presented and discussed in a future publication.
With the linear variable filter in place, the temperature was measured by the thermocouple, and was
found to increase by approximately 1 °C independent of wavelength.
5
Conclusions
Figure 17 shows a preliminary result of the action
spectrum of the Prussian blue tested, that is colour
A novel instrument and a new experimental
method have been presented and employed that
enables the investigation of photosensitive samples and works of art. The instrument demon-
Figure 16: The power variation at focus of the wavelength of tunable microfadometer. An Ocean Optics LVF-UV-HL is used from
405 nm to 475 nm and an LVF-HL filter from 500 nm to 700 nm.
Figure 17: The action spectrum from 405 nm to 700 nm of the
neat Prussian blue sample.
26
Portable Micro-Fading Spectrometer, e-PS, 2008, 5, 17-28
© by M O R A N A RTD d.o.o.
strates increased structural stability increasing the
portability over an earlier design, broadening the
scope of locations at which data can be acquired.
Increased precision of probe positioning relative to
the sample, homogeneity of illumination across the
faded area, controlled intensity at the illuminated
surface from the lamp, and an ease of confocal
probe alignment are present in the new design.
Two different measurement methods indicate a
temperature increase of 3 to 4 degrees during fading experiments.
The instrument is capable of fading Blue Wool 1 to
a ΔE ab value of 7 in just over 5 min and Blue Wool
2 to the same level in just over 10 min.
Incorporating a linear variable filter enables the
investigation of the wavelength dependence of
fading of many samples to a greater resolution
than previously attempted.
It was also found that colour change is not the only
factor that increases the visibility of faded spots.
The smoothness and uniformity of a surface also
plays a role, leading to the conclusion that damage
is hidden by the texture of its surroundings. When
fading very smooth and uniform surfaces colour
change was observable in the case of very small
differences. Therefore under certain circumstances, greater caution should be employed when
the prevention of visible bleaching is a consideration of the fading process.
Investigation of the error in colour measurement,
and colour difference calculations produced by
small differences in position, indicated that alignment by eye may be a significant cause of error in
measurements. A motorized micrometer stage
used for fine adjustment of the focus can reduce
the errors.
Suitable future efforts will be in creating an automated focusing method which would be more suitable for increased accuracy and repeatability of
measurements. Following this, improvements to
create full automation of the instrument will take
place in order to characterize a large number of
samples many times without human intervention.
In doing so, it will be possible to gain large
amounts of data for single samples, and therefore
errors due to such small sampling area can be
reduced by averaging over a very large number of
fades when investigating samples rather than real
art works. Larger sample sets and increased spot
size are also desirable in order to increase the reliability of future wavelength dependent investigations.
6
Acknowledgments
This research is funded by the Public Sector
Research Exploitation Fund (PSRE) which sponsors the project at Tate.
Andrew Lerwill’s thanks and appreciation go to the
Tate Conservation Department for support of the
project, to Anna Brookes for her hard work, to The
Nottingham Trent University School of Science
and Technology for the use of facilities. Thanks
also go to Dr. Gareth Cave of Nottingham Trent
University for his ideas and constructive criticism.
7
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